Siloxanes are used in numerous technical applications because of their unique properties such as water repellency, interface activity, temperature stability, etc. These include the stabilization of polyurethane foams, use as emulsifiers, in release coatings and many others.
However, in order to be able to utilize the unique properties of siloxanes in technical applications, it is usually necessary to modify the siloxane with organic groups, since the pure silicone is generally incompatible with aqueous or organic formulations.
In order to bond organic groups to a siloxane, there are in principle two possible binding types available. In the first case, a carbon atom is bonded directly to a silicon atom (SiC bond formation); in the second case, a carbon atom is bonded to the silicon atom via an oxygen atom (SiOC bond formation). The SiC bond formation usually results from a hydrosilylation reaction, while there are several possible methods available for the formation of an SiOC bond. Classically, SiOC bonds are formed by the reaction of a siloxane with a leaving group (for example halogen) bonded to the silicon atom and an alcohol. Particularly chlorosiloxanes are widely used for this reaction type. However, chlorosiloxanes are difficult to handle, since they are extremely reactive. The use of chlorosiloxanes is also associated with the disadvantage that the hydrogen chloride formed in the course of the reaction restricts handling to corrosion-resistant plants and leads to ecological problems. In addition, organic chlorine compounds may be formed in the presence of chlorosiloxanes and alcohols and are not desirable for toxicological reasons. In addition, it is not easy to achieve a quantitative conversion in the reaction of a chlorosiloxane with an alcohol. Frequently, bases which serve as HCl scavengers have to be used in order to achieve good conversions. The use of these bases results in the formation of large amounts of salt burden which in turn cause problems in their removal on the industrial scale.
A possible alternative to this process is to react alcohols with siloxanes in which hydrogen is bonded directly to the silicon atom (SiH siloxanes). Under suitable conditions, the SiOC bond is formed only with elimination of hydrogen and there is no salt burden. This dehydrogenative condensation proceeds only in the presence of a catalyst. U.S. Pat. No. 5,147,965 refers to a process which is described in the Japanese patent specification JP-A-4-819941 and in which an SiH siloxane is reacted with an alcohol with the addition of alkali metal hydroxides or alkali metal alkoxides. A disadvantage of this process which is mentioned is that although these conditions are suitable for catalyzing a dehydrogenative condensation, they result equally in equilibration and therefore rearrangement of the siloxane basic structure. If the intention is not to change the siloxane basic structure in the course of the reaction, this method is unsuitable. In contrast, EP-B-0 475 440 describes a process in which SiH siloxanes are reacted with an alcohol with the addition of an organic acid in the presence of a Pt salt. Under these conditions, there is no rearrangement of the siloxane basic structure. However, it is unavoidable for the reaction that large amounts of organic acid (from 0.1 to 1 mol based on the alcohol), toluene as a solvent and a platinum salt are used. Since both the toluene and the organic acid are undesired in the end product, they have to be removed on completion of reaction. Platinum salts are not only costly, but are also not entirely safe from a physiological point of view. Specifically in the field of the cosmetics industry, there is a desire for products which are free of platinum.
For the preparation of alkoxysilanes by alcoholyzing monomeric hydrosilanes, the literature describes the heterogeneous catalysis of salts, for example potassium tartrate, phthalate or formate. The reactions require the equimolar use of the salt (based on SiH units) and only succeed at high temperatures of approx. 180° C. (J. Boyer, R. J. P. Corriu, R. Perz, C. Reye J. Organomet. Chem. 1978, 157, 153–162). Both the severe conditions and the large amounts of salt required make the reaction unattractive for the industrial scale.
Very recently, the literature has reported two further methods for dehydrogenative condensation of monomeric hydrosilanes with alcohols. Firstly, tris(pentafluorophenyl)borane may be used as the catalyst for the reaction, in which case from 1 to 8 mol % of the compound is used as a catalyst and operation is effected in a solvent (J. M. Blackwell, K. L. Foster, V. H. Beck, W. E. Piers J. Org. Chem. 1999, 64, 4887–4892). A disadvantage of this catalyst is that it remains in the reaction product and that its biodegradability is inadequate. In addition, boron compounds are frequently undesired, since they are physiologically active.
Secondly, the reaction can be catalyzed by the Grubbs catalyst Cl2(PCy3)2Ru═CHPh, in which case 0.5 mol % of the compound is used as a catalyst. In this process, operation may be effected without solvent. Disadvantages of this catalyst are the high cost and the fact that it is very sensitive toward oxidation, so that operation has to be effected under rigorous exclusion of air. In addition, the catalyst is active not only in the desired dehydrogenative condensation, but also in the homogeneously catalyzed hydrogenation, so that the hydrogen formed during the condensation can hydrogenate double bonds present in the substrate (S. V. Maifeld, R. L. Miller, D. Lee Tetrahedron Lett. 2002, 43, 6363–6366).
There is therefore a need to find a technically simple process which allows siloxanes to be selectively reacted with alcohols, without degrading the siloxane basic structure, without chlorine and without solvent, working with an inexpensive catalyst system which is physiologically safe and can be removed in a simple manner.
In the effort to overcome the disadvantages of the prior art and to provide a process which enables an advantageous alternative preparation of alkoxy-modified siloxanes, it has now been found that this aim can be achieved by the reaction of an SiH siloxane with an alcohol in the presence of a small amount of a catalytic mixture which consists of at least one acid and at least one salt of an acid.